Weak Antilocalization Measurements Research Articles

High transparency induced superconductivity in field effect two-dimensional electron gases in undoped InAs/AlGaSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in 24 nm wide indium arsenide surface quantum wells. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 2 in magnetic fields of up to B = 18 T, at electron densities up to 8 ×1011/cm2. Peak mobility is 11 000 cm2/Vs at 2 ×1012/cm2. Large Rashba spin–orbit coefficients up to 124 meV Å are obtained through weak anti-localization measurements. Proximitized superconductivity is demonstrated in Nb-based superconductor-normal-superconductor (SNS) junctions, yielding 78%–99% interface transparencies from superconducting contacts fabricated ex situ (post-growth), using two commonly used experimental techniques for measuring transparencies. These transparencies are on a par with those reported for epitaxially grown superconductors. These SNS junctions show characteristic voltages IcRn up to 870 μV and critical current densities up to 9.6 μA/μm, among the largest values reported for Nb-InAs SNS devices.

Spin-Orbit Torques and Spin Hall Magnetoresistance Generated by Twin-Free and Amorphous Bi0.9 Sb0.1 Topological Insulator Films.

Topological insulators have attracted great interest as generators of spin-orbit torques (SOTs) in spintronic devices. Bi1-x Sbx is a prominent topological insulator that has a high charge-to-spin conversion efficiency. However, the origin and magnitude of the SOTs induced by current-injection in Bi1-x Sbx remain controversial. Here, the investigation of the SOTs and spin Hall magnetoresistance resulting from charge-to-spin conversion in twin-free epitaxial layers of Bi0.9 Sb0.1 (0001) coupled to FeCo are investigated, and compared with those of amorphous Bi0.9 Sb0.1 . A large charge-to-spin conversion efficiency of 1 in the first case and less than 0.1 in the second is found, confirming crystalline Bi0.9 Sb0.1 as a strong spin-injector material. The SOTs and spin Hall magnetoresistance are independent of the direction of the electric current, indicating that charge-to-spin conversion in single-crystal Bi0.9 Sb0.1 (0001) is isotropic despite the strong anisotropy of the topological surface states. Further, it is found that the damping-like SOT has a non-monotonic temperature dependence with a minimum at 20K. By correlating the SOT with resistivity and weak antilocalization measurements, charge-spin conversion is concluded to occur via thermally excited holes from the bulk states above 20K, and conduction through the isotropic surface states with increasing spin polarization due to decreasing electron-electron scattering below20K.

Gate-defined two-dimensional hole and electron systems in an undoped InSb quantum well

Quantum transport measurements are performed in gate-defined, high-quality, two-dimensional hole and electron systems in an undoped InSb quantum well. For both polarities, the carrier systems show tunable spin-orbit interaction as extracted from weak antilocalization measurements. The effective mass of InSb holes strongly increases with carrier density as determined from the temperature dependence of Shubnikov--de Haas oscillations. Coincidence measurements in a tilted magnetic field are performed to estimate the spin susceptibility of the InSb two-dimensional hole system. The $g$ factor of the two-dimensional hole system decreases rapidly with increasing carrier density.

Field effect two-dimensional electron gases in modulation-doped InSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in surface indium antimonide quantum wells. The topmost 5 nm of the 30 nm wide quantum well is doped and shown to promote the formation of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 1 in magnetic fields of up to B = 18 T. We show that the electron density is gate-tunable, reproducible, and stable from pinch-off to 4 ×1011 cm−2, and peak mobilities exceed 24 000 cm2/V s. Large Rashba spin–orbit coefficients up to 110 meV ·Å are obtained through weak anti-localization measurements. An effective mass of 0.019me is determined from temperature-dependent magnetoresistance measurements, and a g-factor of 41 at a density of 3.6 ×1011 cm−2 is obtained from coincidence measurements in tilted magnetic fields. By comparing two heterostructures with and without a delta-doped layer beneath the quantum well, we find that the carrier density is stable with time when doping in the ternary Al0.1In0.9Sb barrier is not present. Finally, the effect of modulation doping on structural asymmetry between the two heterostructures is characterized.

Spin-Orbit Interaction in GaN / Alx Ga1−x N Heterojunctions Probed by Electron Spin Resonance

Fundamental aspects of spin-orbit interaction in commercially relevant $\mathrm{Ga}\mathrm{N}$/${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ heterostructures hosting two-dimensional electron systems are extensively studied by electron spin resonance (ESR). This unprecedentedly accurate experimental technique allows access to the fine details of coupling between the spin degree of freedom and the quantized orbital motion of an electron in the quantum Hall regime through the precise measurement of the electron $g$-factor. Filling-factor-dependent changes of the $g$-factor value in strong magnetic fields allow extraction of the Rashba spin-orbit interaction constant $\ensuremath{\alpha}$ in various $\mathrm{Ga}\mathrm{N}$/${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ heterojunctions with electron sheet densities in the range $0.8--5.2\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Despite three significantly different approaches used to tune the electron density, the extracted value of $\ensuremath{\alpha}$ is on the order of $5.3\ifmmode\pm\else\textpm\fi{}0.4$ meV $\text{\AA{}}$ for all experimental realizations. This striking finding can be explained by assuming that the spin-orbit interaction is of bulk origin. Theoretical calculations confirm this observation, as the bulk cubic in-plane wave-vector term of the spin-orbit interaction compensates the rising contribution due to the change in the quantum well shape. Finally, the value of $\ensuremath{\alpha}$ is cross-checked and confirmed by weak antilocalization measurements in the longitudinal magnetoresistance at substantially lower magnetic field values compared to the ESR approach. The presented experimental findings provide knowledge for estimating the true scales of spin-orbit interaction in GaN-based spintronic devices.

Tunable spin-orbit coupling in two-dimensional InSe

We demonstrate that spin-orbit coupling (SOC) strength for electrons near the conduction band edge in few-layer $\gamma$-InSe films can be tuned over a wide range. This tunability is the result of a competition between film-thickness-dependent intrinsic and electric-field-induced SOC, potentially, allowing for electrically switchable spintronic devices. Using a hybrid $\mathbf{k\cdot p}$ tight-binding model, fully parameterized with the help of density functional theory computations, we quantify SOC strength for various geometries of InSe-based field-effect transistors. The theoretically computed SOC strengths are compared with the results of weak antilocalization measurements on dual-gated multilayer InSe films, interpreted in terms of Dyakonov-Perel spin relaxation due to SOC, showing a good agreement between theory and experiment.

Extraordinary phase coherence length in epitaxial halide perovskites

SummaryInorganic halide perovskites have emerged as a promising platform in a wide range of applications from solar energy harvesting to computing and light emission. The recent advent of epitaxial thin film growth of halide perovskites has made it possible to investigate low-dimensional quantum electronic devices based on this class of materials. This study leverages advances in vapor-phase epitaxy of halide perovskites to perform low-temperature magnetotransport measurements on single-domain cesium tin iodide (CsSnI3) epitaxial thin films. The low-field magnetoresistance carries signatures of coherent quantum interference effects and spin-orbit coupling. These weak anti-localization measurements reveal a micron-scale low-temperature phase coherence length for charge carriers in this system. The results indicate that epitaxial halide perovskite heterostructures are a promising platform for investigating long coherent quantum electronic effects and potential applications in spintronics and spin-orbitronics.

Determination of spin diffusion length in Bi [formula omitted]-doped Cu films by weak antilocalization measurements

Working with the weak antilocalization measurements, we investigate the spin diffusion length in Bi δ-doped Cu films with the fine-tuned Bi concentration. By exploiting the long spin diffusion length of Cu and the strong spin-orbit coupling of Bi with the δ-doping technique, the spin diffusion length can be tuned artificially. Furthermore, it is found that the spin relaxation is dominated by elastic scatterings from Bi impurities in Bi δ-doped Cu systems, which follows the Elliott-Yafet mechanism.

Spin diffusion length in Au wires and films investigated by weak antilocalization measurements

Working with weak antilocalization measurements on Au wires and films grown by molecular beam epitaxy and dc magnetron sputtering with fine-tuned electron scatterings, we investigated the spin diffusion length in Au mesoscopic structures in quasi-1D and quasi-2D regimes. The result shows that the spin relaxations in both Au quasi-1D wires and quasi-2D films follow the Elliott–Yafet mechanism regardless of the deposition method. Furthermore, we observed a dimensionality behavior of spin diffusion between the quasi-1D and quasi-2D cases following the generalized Einstein's equation on Brownian motion.

Transport properties of near surface InAs two-dimensional heterostructures

Two-dimensional electron systems confined to the surface of narrowband semiconductors have attracted great interest since they can easily integrate with superconductivity (or ferromagnetism) enabling new possibilities in hybrid device architectures and study of exotic states in proximity to superconductors. In this work, we study indium arsenide heterostructures where a combination of a clean interface with superconductivity, high mobility, and spin-orbit coupling can be achieved. The weak antilocalization measurements indicate the presence of strong spin-orbit coupling at high densities. We study the magnetotransport as a function of top barrier and density and report a clear observation of integer quantum Hall states. We report improved electron mobility reaching up to 44 000 cm2/Vs in undoped heterostructures and well developed integer quantum Hall states starting as low as 2.5 T.

Helical Hole State in Multiple Conduction Modes in Ge/Si Core/Shell Nanowire.

Helical states, a prerequisite for the engineering of Majorana zero modes in solid-state systems, have recently been reported in the conduction band of III-V nanowires (NWs) subject to strong Rashba spin-orbit interaction. We report the observation of re-entrant conductance features consistent with the presence of helical hole states in multiple conduction modes of a Ge/Si core/shell NW. The Ge/Si system has several potential advantages over electron systems such as longer spin coherence time due to weaker coupling to nuclear spins and the possibility of isotope-purified materials for nuclear spin-free devices. We derive the Landé g factor of 3.6 from magneto-transport measurements, comparable to theoretical predictions and significantly larger when compared with that in strongly confined quantum dots. The spin-orbit energy is evaluated as ∼2.1 meV, on par with values in III-V NWs, showing good agreement with previous theoretical predictions and weak antilocalization measurements.

Link between the Superconducting Dome and Spin-Orbit Interaction in the (111) LaAlO_{3}/SrTiO_{3} Interface.

We measure the gate voltage (V_{g}) dependence of the superconducting properties and the spin-orbit interaction in the (111)-oriented LaAlO_{3}/SrTiO_{3} interface. Superconductivity is observed in a dome-shaped region in the carrier density-temperature phase diagram with the maxima of superconducting transition temperature T_{c} and the upper critical fields lying at the same V_{g}. The spin-orbit interaction determined from the superconducting parameters and confirmed by weak-antilocalization measurements follows the same gate voltage dependence as T_{c}. The correlation between the superconductivity and spin-orbit interaction as well as the enhancement of the parallel upper critical field, well beyond the Chandrasekhar-Clogston limit, suggest that superconductivity and the spin-orbit interaction are linked in a nontrivial fashion. We propose possible scenarios to explain this unconventional behavior.

Origin and Magnitude of ‘Designer’ Spin-Orbit Interaction in Graphene on Semiconducting Transition Metal Dichalcogenides

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultra-strong and extremely robust, despite it being merely interfacially-induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, and of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization measurements as the simplest and most general diagnostic of SOI, and show that the spin relaxation time is very short in all cases regardless of the elastic scattering time. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov-de Haas magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$. These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI, as well as its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.

Experimental observation on a temperature-induced decoupling between the surface states in topological insulator nanoplates Bi2−0.15(TeSe)3+0.15

We report on the temperature-induced quantum decoupling between the surface states, demonstrated by weak antilocalization measurements, in the topological insulator Bi2−0.15(TeSe)3+0.15 nanoplates. The samples are prepared by a catalyst-free vapor–solid process with the dimension of 10 μm and the thickness of 20 nm. The channel indicator is extracted from the weak antilocalization, which presents a transition from 0.5 to 1 with the increasing temperature. This reveals the coherent decoupling between the two surface states, during which the dephasing length reaches the plate thickness. A bulk-mediated intersurface coupling model interprets the transition.

Conductance Change Induced by the Rashba Effect in the LaAlO<SUB>3</SUB>/SrTiO<SUB>3</SUB> Interface

The LaAlO3/SrTiO3 (LAO/STO) heterostructure has an inherent space inversion asymmetry causing an internal electric field near the interface. The Rashba spin-orbit coupling arising from this structural characteristic has a considerable influence on spin transport. With application of an external magnetic field, we observed conductance change in the LAO/STO interface which depends on the sign and magnitude of the field. Our systematic study revealed that these results come from spin dependent transport, by which we obtained quantitative strength of the Rashba effect. The Rashba strength in this system depends on the temperature: it varies from 2.6 x 10(-12) eVm to negligible value in the temperature range of 1.8 K-12 K. This method for detecting Rashba effect covers a wider temperature range in comparison with those obtained from Shubnikov-de Haas oscillation or weak antilocalization measurements.

Experimental Verification of Comparability between Spin-Orbit and Spin-Diffusion Lengths

We experimentally confirmed that the spin-orbit lengths of noble metals obtained from weak antilocalization measurements are comparable to the spin diffusion lengths determined from lateral spin valve ones. Even for metals with strong spin-orbit interactions such as Pt, we verified that the two methods gave comparable values which were much larger than those obtained from recent spin torque ferromagnetic resonance measurements. To give a further evidence for the comparability between the two length scales, we measured the disorder dependence of the spin-orbit length of copper by changing the thickness of the wire. The obtained spin-orbit length nicely follows a linear law as a function of the diffusion coefficient, clearly indicating that the Elliott-Yafet mechanism is dominant as in the case of the spin diffusion length.

Energy relaxation probed by weak antilocalization measurements in GaN heterostructures

Energy relaxation and electron-phonon (e-p) interaction are investigated in wurtzite Al0.15Ga0.85N/AlN/GaN and Al0.83In0.17N/AlN/GaN heterostructures with polarization induced two-dimensional electron gases in the Bloch–Grüneisen regime. Weak antilocalization (WAL) and Shubnikov–de Haas measurements were performed on gated Hall bar structures at temperatures down to 0.3 K. We used WAL as a thermometer to measure the electron temperature Te as a function of the dc bias current. We found that the power dissipated per electron, Pe, was proportional to Te4 due to piezoelectric acoustic phonon emission by hot electrons. We calculated Pe as a function of Te without any adjustable parameters for both the static and the dynamic screening cases of piezoelectric e-p coupling. In the temperature range of this experiment, the static screening case was expected to be applicable; however, our data was in better agreement with the dynamic screening case.

Strong spin-orbit interactions and weak antilocalization in carbon-dopedp-typeGaAs∕AlxGa1−xAsheterostructures

We present a comprehensive study of the low-field magnetoresistance in carbon-doped $p$-type $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ heterostructures aiming at the investigation of spin-orbit interaction effects. The following signatures of exceptionally strong spin-orbit interactions are simultaneously observed: a beating in the Shubnikov--de Haas oscillations, a classical positive magnetoresistance due to the presence of the two spin-split subbands, and a weak antilocalization dip in the magnetoresistance. The spin-orbit-induced splitting of the heavy hole subband at the Fermi level is determined to be around 30% of the total Fermi energy. The phase-coherence length of holes of around $2.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at a temperature of $70\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, extracted from weak antilocalization measurements, is promising for the fabrication of phase-coherent $p$-type nanodevices.

Measurement of linear and cubic spin–orbit coupling parameters in AlGaN/AlN/GaN heterostructures with a polarization-induced two-dimensional electron gas

Spin–orbit coupling is investigated by magneto-conductivity measurements in Al x Ga 1– x N/AlN/GaN heterostructures with a polarization-induced two-dimensional electron gas having a carrier density range from 0.8×10 12 cm −2 to 10.6×10 12 cm −2. The zero-field electron spin-splitting energies extracted from the weak antilocalization measurements are found to scale with the Fermi wavevector k F as 2 ( α k F + γ k F 3 ) with effective linear and cubic spin–orbit parameters of α=5.0±0.3×10 −13 eV m 3 and γ=1.6±0.6×10 −31 eV m 3, respectively. The cubic spin–orbit coupling parameter is purely due to the bulk inversion asymmetry of the wurtzite crystal and has not been previously measured for the GaN system.

Suppression of weak antilocalization in anAlxGa1−xN∕GaNtwo-dimensional electron gas by an in-plane magnetic field

We studied the suppression of weak antilocalization in an ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ two-dimensional electron gas in the presence of an additional in-plane magnetic field. By comparing our experimental data to a theoretical model, we concluded that the suppression can be attributed mainly to the Zeeman effect, while the contribution due to disorder at the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterointerface is considerably smaller. Furthermore, our results give further evidence for the value of spin-orbit scattering length determined from weak antilocalization measurements.